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U.S. Department of the Interior / U.S. Geological SurveyVolcanoesThe eruption of Cerro Negro Volcano, near Leon, Nicaragua, during November 1968.Volcanoesby Robert I. TillingCover and Title Page: Lava fountains and flows, Mauna Loa, Hawaii, July 6, 1975.Volcanoes destroy and volcanoes create. The catastrophic eruption of Mount St. Helens on May 18, 1980, made clear the awesome destructive power of a volcano. Yet, over a time span longer than human memory and record, volcanoes have played a key role in form ing and modifying the planet upon which we live. More than 80 per cent of the Earth s surface above and below sea level is of volcanic origin. Gaseous emissions from volcanic vents over hundreds of millions of years formed the Earth s earliest oceans and atmosphere, which supplied the ingredients vital to evolve and sustain life. Over geologic eons, countless volcanic eruptions have produced moun tains, plateaus, and plains, which subsequent erosion and weathering have sculpted into majestic landscapes and formed fertile soils.Ironically, these volcanic soils and inviting terranes have attracted, and continue to attract, people to live on the flanks of volcanoes. Thus, as population density increases in regions of active or potential ly active volcanoes, mankind must become increasingly aware of the hazards and learn not to “crowd“ the volcanoes. People living in the shadow of volcanoes must live in harmony with them and expect, and should plan for, periodic violent unleashings of their pent-up energy.This booklet presents a generalized summary of the nature, work ings, products, and hazards of the common types of volcanoes around the world, along with a brief introduction to the techniques of volcano monitoring and research.On August 24, A.D. 79, Vesuvius Volcano suddenly exploded and destroyed the Roman cities of Pompeii and Herculaneum. Al though Vesuvius had shown stir rings of life when a succession of earthquakes in A.D. 63 caused some damage, it had been literally quiet for hundreds of years and was considered “extinct.“ Its surface and crater were green and covered with vegetation, so the eruption was totally unexpected. Yet in a few hours, hot volcanic ash and dust buried the two cities so thoroughly that their ruins were not uncovered for nearly 1,700 years, when the discovery of an outer wall in 1748 started a period of modern arche ology. Vesuvius has continued its activity intermittently ever since A.D. 79 with numerous minor eruptions and several major eruptions occur ring in 1631, 1794, 1872, 1906 and in 1944 in the midst of the Italian campaign of World War II.In the United States on March 27, 1980, Mount St. Helens Volcano in the Cascade Range, southwestern Washington, reawakened after more than a century of dormancy and provided a dramatic and tragic reminder that there are active volcanoes in the “lower 48“ States as well as in Hawaii and Alaska. The catastrophic eruption of Mount St. Helens on May 18, 1980, and related mudflows and flooding caused significant loss of life (57 dead or missing) and property damage (over $1.2 billion). Mount St. Helens is expected to remain in termittently active for months or years, possibly even decades.The word “volcano“ comes from the little island of Vulcano in the Mediterranean Sea off Sicily. Cen turies ago, the people living in this area believed that Vulcano was the chimney of the forge of Vulcan the blacksmith of the Roman gods. They thought that the hot lava frag ments and clouds of dust erupting from Vulcano came from Vulcan s forge as he beat out thunderbolts for Jupiter, king of the gods, and weapons for Mars, the god of war. In Polynesia the people attributed eruptive activity to the beautiful but wrathful Pele, Goddess of Volca noes, whenever she was angry or spiteful. Today we know that volcanic eruptions are not super natural but can be studied and in terpreted by scientists.The Nature of VolcanoesVolcanoes are mountains, but they are very different from other mountains; they are not formed by folding and crumpling or by uplift and erosion. Instead, volcanoes are built by the accumulation of their own eruptive products lava, bombs (crusted over lava blobs), ashflows, and tephra (airborne ash and dust). A volcano is most commonly a con ical hill or mountain built around a vent that connects with reservoirs of molten rock below the surface of the Earth. The term volcano also refers to the opening or vent through which the molten rock and associated gases are expelled.Driven by buoyancy and gas pres sure the molten rock, which is lighter than the surrounding solid rock, forces its way upward and may ulti mately break through zones of weak nesses in the Earth s crust. If so, an eruption begins, and the molten rock may pour from the vent as non- explosive lava flows, or it may shoot violently into the air as dense clouds of lava fragments. Larger fragments fall back around the vent, and ac cumulations of fall-back fragments may move downslope as ash flows under the force of gravity. Some of the finer ejected materials may be carried by the wind only to fall to the ground many miles away. The finest ash particles may be injectedFountaining lava and volcanic debris during the 1959 Kilauea Iki eruption of Kilauea Volcano, Hawaii.miles into the atmosphere and car ried many times around the world by stratospheric winds before settling out.Molten rock below the surface of the Earth that rises in volcanic vents is known as magma, but after it erupts from a volcano it is called lava. Originating many tens of miles beneath the ground, the ascending magma commonly contains some crystals, fragments of surrounding (unmelted) rocks, and dissolved gases, but it is primarily a liquid composed principally of oxygen, silicon, aluminum, iron, magnesium, calcium, sodium, potassium, titan ium, and manganese. Magmas also contain many other chemical ele ments in trace quantities. Upon cooling, the liquid magma may pre cipitate crystals of various minerals until solidification is complete to form an igneous or magmatic rock.The diagram below shows that heat concentrated in the Earth s up per mantle raises temperatures suf ficiently to melt the rock locally by fusing the materials with the lowest melting temperatures, resulting in small, isolated blobs of magma. These blobs then collect, rise through conduits and fractures, and some ultimately may re-collect in larger pockets or reservoirs (“hold ing tanks“) a few miles beneath the Earth s surface. Mounting pressure within the reservoir may drive the magma further upward through structurally weak zones to erupt as lava at the surface. In a continental environment, magmas are genera ted in the Earth s crust as well as at varying depths in the upper man tle. The variety of molten rocks in the crust, plus the possibility of mix ing with molten materials from the underlying mantle, leads to theOceanic TrenchOCEANIC CRUSTCONTINENTAL CRUSTPlate BoundaryBASALTIC MAGMALOW VELOCITY ZONEAn idealized diagram of a volcano in an oceanic environment (left) and in a continental environment (right).production of magmas with widely different chemical compositions.If magmas cool rapidly, as might be expected near or on the Earth s surface, they solidify to form igneous rocks that are finely crystalline or glassy with few crystals. Accordingly, lavas, which of course are very rapidly cooled, form volcanic rocks typically characterized by a small percentage of crystals or fragments set in a matrix of glass (quenched or super-cooled magma) or finer grained crystalline materials. If magmas never breach the surface to erupt and remain deep under ground, they cool much more slowly and thus allow ample time to sustain crystal precipitation and growth, re sulting in the formation of coarser grained, nearly completely crystal line, igneous rocks. Subsequent to final crystallization and solidification, such rocks can be exhumed by ero sion many thousands or millions of years later and be exposed as large bodies of so-called granitic rocks, as, for example, those spectacularly displayed in Yosemite National Park and other parts of the majestic Sierra Nevada mountains of California.Lava is red hot when it pours or blasts out of a vent but soon chang es to dark red, gray, black, or some other color as it cools and solidifies. Very hot, gas-rich lava containing abundant iron and magnesium is fluid and flows like hot tar, whereas cooler, gas-poor lava high in silicon, sodium, and potassium flows slug gishly, like thick honey in some cases or in others like pasty, blocky masses.All magmas contain dissolved gases, and as they rise to the sur face to erupt, the confining pressuresare reduced and the dissolved gases are liberated either quietly or explo sively. If the lava is a thin fluid (not viscous), the gases may escape easily. But if the lava is thick and pasty (highly viscous), the gases will not move freely but will build up tre mendous pressure, and ultimately escape with explosive violence. Gases in lava may be compared with the gas in a bottle of a carbon ated soft drink. If you put your thumb over the top of the bottle and shake it vigorously, the gas separates from the drink and forms bubbles. WhenTwo Polynesian terms are used to identify the surface character of Hawaiian lava flows. Aa, a basalt with a rough, blocky appear ance, much like furnace slag, is shown at the top. Pahoehoe, a more fluid variety with a smooth, satiny and sometimes glassy ap pearance, is shown at the bottom.you remove your thumb abruptly, there is a miniature explosion of gas and liquid. The gases in lava behave in somewhat the same way. Their sudden expansion causes the terrible explosions that throw out great masses of solid rock as well as lava, dust, and ashes.The violent separation of gas from lava may produce rock froth calledpumice. Some of this froth is so light because of the many gas bubbles that it floats on water. In many eruptions, the froth is shat tered explosively into small frag ments that are hurled high into the air in the form of volcanic cinders (red or black), volcanic ash (com monly tan or gray), and volcanic dust.During the 1959 eruption of Kilauea Iki, fountaining lava and volcanic debris completely blocked several of the roads in the Hawaii Volcanoes National Park.Principal Types of VolcanoesGeologists generally group vol canoes into four main kinds cinder cones, composite volcanoes, shield volcanoes, and lava domes.Cinder conesCinder cones are the simplest type of volcano. They are built from particles and blobs of congealed lava ejected from a single vent. As the gas-charged lava is blown vio lently into the air, it breaks into small fragments that solidify and fall as cinders around the vent to form a circular or oval cone. Most cinder cones have a bowl-shaped crater atthe summit and rarely rise more than a thousand feet or so above their surroundings. Cinder cones are numerous in western North America as well as throughout other volcanic terrains of the world.In 1943 a cinder cone started growing on a farm near the village of Parfcutin in Mexico. Explosive eruptions caused by gas rapidly ex panding and escaping from molten lava formed cinders that fell back around the vent, building up the cone to a height of 1,200 feet. The last explosive eruption left a funnel- shaped crater at the top of theSchematic representation of the internal structure of a typical cinder cone. 8cone. After the excess gases had largely dissipated, the molten rock quietly poured out on the surround ing surface of the cone and moved downslope as lava flows. This order of events eruption, formation of cone and crater, lava flow is a common sequence in the formation of cinder cones.During 9 years of activity, Parfcutin built a prominent cone, covered about 100 square miles with ashes, and destroyed the town of San Juan. Geologists from many parts of the world studied Parfcutin during its lifetime and learned a great deal about volcanism, its products, and the modification of a volcanic land- form by erosion.Parfcutin Volcano, Mexico, is a cinder cone rising approximately 1,200 feet above the surrounding plain.Composite volcanoesSome of the Earth s grandest mountains are composite volcanoes sometimes called stratovolcanoes. They are typically steep-sided, symmetrical cones of large dimension built of alternating layers of lava flows, volcanic ash, cinders, blocks, and bombs and may rise as much as 8,000 feet above their bases. Some of the most conspicuous and beautiful mountains in the world are com posite volcanoes, including Mount Fuji in Japan, Mount Cotopaxi in Ecuador, Mount Shasta in Califor nia, Mount Hood in Oregon, andMount St. Helens and Mount Rainier in Washington.Most composite volcanoes have a crater at the summit which contains a central vent or a clustered group of vents. Lavas either flow through breaks in the crater wall or issue from fissures on the flanks of the cone. Lava, solidified within the fissures, forms dikes that act as ribs which greatly strengthen the cone.The essential feature of a com posite volcano is a conduit system through which magma from a reser voir deep in the Earth s crust rises to the surface. The volcano is built up by the accumulation of materialSchematic representation of the internal structure of a typical composite volcano. 10erupted through the conduit and in creases in size as lava, cinders, ash, etc., are added to its slopes. When a composite volcano be comes dormant, erosion begins to destroy the cone. As the cone is stripped away, the hardened mag ma filling the conduit (the volcanic plug) and fissures (the dikes) becomes exposed, and it too is slowly reduced by erosion. Finally, all that remains is the plug and dike complex projecting above the land surface a telltale remnant of the vanished volcano.An interesting variation of a com posite volcano can be seen at Crater Lake in Oregon. From what geologists can interpret of its past, a high volcano called Mount Mazama probably similar in ap pearance to present-day Mount Rainier was once located at this spot. Following a series of tremen dous explosions about 6,800 years ago, the volcano lost its top. Enor mous volumes of volcanic ash and dust were expelled and swept down the slopes as ash flows and avalan ches. These large-volume explo-Shishaldin Volcano, an imposing composite cone, towers 9,372 feet above sea level in the Aleutian Islands, Alaska.11sions rapidly drained the lava be neath the mountain and weakened the upper part. The top then col lapsed to form a large depression, which later filled with water and is now completely occupied by beau tiful Crater Lake. A last gasp of eruptions produced a small cinder cone, which rises above the water surface as Wizard Island near the rim of the lake. Depressions such as Crater Lake, formed by collapse of volcanoes, are known as cal- deras. They are usually large, steep-walled, basin-shaped depres sions formed by the collap